Mike,
I halfway know a little something about cast iron ....
If you were dealing with a more solid part, like a big solid flywheel, or just anything that did not have so many intersections of thick and thin and critical mating surfaces and alignment (like an engine block for example) the preheating, welding and controlled cooling necessary for a good weld can be accomplished without much worry as to how the part will function in use later. Moreover, the type of preheat and welding I'm referring to is to heat the entire casting to a RED-HOT condition and weld with a cast-iron electrode. This method produces a weld which is as close as possible to the characteristics of the original cast metal.
Now on a part like an engine block for example with so many intersections of thick and thin iron, the expansion and contraction of these areas when preheated red-hot and cooled will not be exactly the same no matter how they do it; with the thin areas heating and cooling faster and expanding and contracting in conflict with the thick areas. The end result is critical mating areas, machined surfaces, etc. can warp out of alignment.
Now as Pete said above, ALL these areas would have to be re-machined and in my opinion, you'd be damn lucky if every surface and alignment came out correctly as original.
Even if the expert doing the work said everything was correct, from that point forward if you ever had a problem with bearings going out prematurely, premature odd wear on parts, binding, etc., etc., you'd cuss that block forever!
Larry B.
P.S. If the cracks are just in the water jacket and in an area that will not be subject to a lot of load or shock, rods with a nickel base can be used to make welds without all the preheat mentioned above. BUT ...... the casting must be clean, i.e., no oil or contaminants, NOTHING, along with the crack beveled out and the weld made in a downhand position. Now this method is not perfect and can still crack but it often works well and holds up.
